1. Field of the Invention
The present invention relates to a system for dynamically shifting beacons in a distributed wireless network and a method thereof, and more particularly to a system for dynamically shifting beacons in a distributed wireless network and a method thereof that enable respective devices in the network to effectively send their beacons in a medium access control for the wireless personal area network based on a wireless mobile ad-hoc network.
2. Description of the Related Art
A WPAN (Wireless Personal Area Network) is defined as a network that operates in a personal area of about 10 meters. IEEE (Institute of Electrical and Electronics Engineers) is involved in defining standards for such wireless personal area networks. The UWB (Ultra Wide Band) communication technology can provide transmission rates of more than several hundred Mbps in such a personal area. In a WPAN, mediums are shared among all devices for mutual communications. This requires a medium access control method for controlling the medium access of the devices, which includes, in a broad sense, how to access the network, how to transmit data to other devices at a desired transmission rate, how to optimally use the medium, how to detect and dissolve collision of beacons, etc.
The medium access control method for a WPAN may be classified into a centralized access method and a distributed access method. According to the centralized access method, a device operates for the whole network in order to manage and control the medium access for all devices. All other devices request the help of a centralized coordinator for their medium access functions such as network joining and channel time allocation.
According to the distributed access method, the medium access operations are evenly distributed over all devices in the network, and all devices share the burden of management of their mutual medium access. Although the IEEE standard refers to the centralized medium access control method, several distributed medium access control methods have been discussed for the WPAN because such methods provide flexibility in terms of mobility to the devices.
FIG. 1 is a view illustrating a WPAN according to the conventional distributed access method.
Referring to FIG. 1, the WPAN includes many devices that are indicated as points. Circles drawn around the respective device indicate transmission and reception ranges of the device, in which beacons of the device are received. Any device in the network is not acting as a dedicated coordinator.
The WPAN based on the distributed access method does not have any centralized coordinator. In the network, a separate dedicated coordinator is not present, but each device serves as light coordinators and they cooperate with one other. Also, devices share information required for performing the medium access control functions such as a channel time allocation, synchronization, power saving, etc., for the data transmission to other devices. This network system is called an ad-hoc distributed wireless personal area network. Each device periodically broadcasts information about their neighbor devices and information about channel times allocated to the neighbor devices.
The distributed medium access control method depends on a timing concept called ‘superframe’. This superframe has a fixed length in time, and is divided into a plurality of time windows that are called ‘time slots’. Some time slots are used for the devices to send beacons, and the remaining slots are used to send data. The slots that send the beacons are called ‘beacon slots’, and the slots that send the data are called ‘data slots’.
The length of a BP (Beacon Period) that is composed of beacon slots may be shorter than the length of a data period. The beacon slots may be distributed through the superframe, or may appear together at the start part of the superframe. In addition, the index of beacon slots may be fixed, or may be variable leading to different configurations to the distributed medium access control method.
FIG. 2 is a view illustrating an example of a conventional superframe structure.
The superframe structure as illustrated in FIG. 2 is based on what is defined by the Multiband OFDM (Orthogonal Frequency Division Multiplexing) Alliance draft version 0.5. This includes several time slots, also called MASs (Medium Access Slots), e.g., a and c in FIG. 2. Some MASs (a in FIG. 2) make a beacon period (b in FIG. 2). Each MAS in beacon period is divided in to 3 beacon slots. The remaining MASs (c in FIG. 2) make a data period (d in FIG. 2). These MASs can be used by a device in the network in order to transfer the data to other devices in the network. Each superframe is 65,536 μs, and each MAS is 256 μs. The length of the beacon period can be variable.
The variable length beacon period means that the length of the beacon period is increased as the number of beaconing devices becomes larger around the device while the length of the beacon period is reduced as the number of beaconing devices becomes fewer around the device. If there is almost no device subject to communication, the variable length of the beacon period makes it possible to greatly secure the data period d for data communication, and is useful in reducing the power consumption.
Information about the superframe is broadcasted through beacons by the devices. Accordingly, neighboring devices of a device can use the information for the next process. The start time of the superframe is determined by the start of the beacon period, and is defined as a BPST (Beacon Period Start Time).
The devices can use the same BPST for the superframe. The medium access slots are numbered in relation to this start time. All devices would receive the beacons during BP of the superframe to obtain the time synchronization.
In the beacons broadcasted by the devices, BPOIE (Beacon Period Occupancy Information Element) is always included. The BPOIE includes beacon information of neighboring devices received in the beacon period b by the device. Upon reception of a beacon, the device stores the sender's DEVID (Device Identifier) with the beacon slot index in which the beacon was received. Then, in the next superframe, the device includes the stored information in the BPOIE of the beacon being broadcast by it. The information of the beacons received during a superframe is included in the BPOIE to be sent in the next superframe. Here, it should be noted that the BPOIE refers to a method for indicating a device's occupancy of the beacon slots in a two-hop area, and is independent of the beaconing procedure.
If the DEVID of a certain device does not show up in the BPOIE of a neighboring device beacon for specified successive number of superframes, this means that the corresponding device will change the corresponding beacon slot to an idle slot in the next superframe.
According to the conventional method, however, there is no way to dynamically reduce the size of the beacon period and, thus, to use a still larger part of the superframe for the data transmission or power saving is not possible.
In an environment in which many devices are used, the beacon slots of a large beacon period that has been used by the devices may become free as the devices leave or power-off. These free slots may be available in middle of the beacon period. However, even if the free slots are located on the middle of the beacon period, the size of the beacon period is kept as it is, and thus the use of the channel time of the superframe becomes inefficient.
Further, because of a large beacon period having a small number of beacons, the devices has to be awake for long time, and this conflicts with the necessity of power saving.